Photos from Mac uploading to iCloud extremely slow

Hi Everyone,

I am moving my old collection of photos from harddisk to iCloud. There are 500,000+ photos. I am doing in batches of 50,000 only to not make it too heavy. I have constant 300 MBPS connection. However, the upload is happening extremely slowly. It took around 6 days for 1250 photos to be uploaded. These old photos are also less in size hence this is difficult to understand why it is stuck.

In parallel, I uploaded the entire 1 TB of photos/videos in Dropbox and Google Photos which finished the full upload in 1.5 days in Dropbox and in 4 days in Google Photos via Drive.

Can you please help what could be wrong?